Coke for use in metallurgical processes or the like is usually made in retort ovens customarily consisting essentially of coking chambers, heating walls and regenerators, all formed from refractory brick. The coking chambers are rectangular, perhaps 30 to 50 feet long, 10 to 20 feet in height and 1 to 2 feet in width. Heating walls are positioned on each side of the coking chambers. A number of alternately side by side coking chambers and heating walls constitute a battery of ovens. The regenerators are located under the coking chambers and heating walls. In operation coal is placed in the coking chambers and heated by gas burned in the flues within the heating walls in regular alternating relationship. Combustion air is preheated in the regenerators. After a number of hours the resulting coke is pushed mechanically from one end of the vertical coking chambers. The coal particles tend to adhere to each other during the heating cycle of the coking process and emerge from the coking chamber in a more or less coherent cake of hot glowing coke which is not strong enough to support its own weight and breaks off into smaller lumps as it is progressively pushed from the oven.
The goal of any coke oven heating system is to afford a coke cake that is thoroughly and uniformly coked throughout its mass when the cake is pushed. The vertical temperature profile of the pushed coke cake should be relatively constant, i.e., the temperature from the top to the bottom of the coke cake should ideally be substantially the same. The variation in temperature that exists along the vertical height of the coke cake is known as the vertical temperature differential. If the vertical temperature profile is not constant, a "green" push may result that not only yields coke of poor quality but also generates substantial emissions which may pollute the environment.
While it is desirable to have a low vertical temperature differential, the construction of the walls between the coking chambers necessitated in order to achieve structural strength and the fact that gas flames vary in intensity along their length have often resulted in coke cakes having a rather large, or high, vertical temperature differential, particularly near the top of the cake.
Prior art proposals to solve this problem endeavored to achieve a low vertical temperature differential within the coking chamber and, thus, within the coke cake itself by providing a heating flue system that has a small temperature drop from the base to the top of the vertical heating flues.
The basic design of the Koppers-Becker heating system, which is shown in various embodiments in U.S. Pat. Nos. 2,100,762; 2,255,406; 2,306,366; 3,192,129 and other patents issued to J. Becker, was an early attempt at a low differential heating system. Groups of adjacent vertical flues communicate with short horizontal flues which are connected by cross-over flues running over the top of the coking chamber to corresponding horizontal and vertical flues in the heating wall on the opposite side of the coking chamber. In actuality, this heating system resulted in a relatively large temperature differential along the height of the vertical flues with a concomitant large vertical temperature differential in the coke mass. Typical vertical temperature profiles of non-multistage heating systems such as the Koppers-Becker heating system are shown in FIG. 6 of "High Coke Ovens" by E. J. Helm, Ironmaking Proceedings, 1966, pp 53-57. The upper part of the coke cake will exhibit the greatest temperature drop since the upper part of the heating flues possess the lowest temperature. To achieve a coke cake which is sufficiently coked in the upper portion would necessitate extended coking cycles and limited coking rates resulting in over-coking of the lower portion of the coke and a waste of fuel. Several methods used currently, with limited success, to alleviate this problem are proportioning the lean gas and/or air between high and low ports within the flues and recirculating part of the waste gases to elongate the flame within the flue.
U.S. Pat. No. 3,801,470 discloses the well-known Firma Carl Still multistage heating system which is a half-divided system utilizing large horizontal flues to conduct the waste gases from the half of the vertical flues which are burning across the battery to the other half through which the waste gases are removed. The vertical temperature profile is relatively constant in the heating flue as well as in that portion of the coke cake up to the level corresponding to the height of the horizontal flue. The heavy brickwork of the horizontal flue, however, apparently interferes with the transfer of heat to the upper part of the coking chamber resulting in a large temperature drop-off in the upper portion of the coke cake.
U.S. Pat. No. 3,953,299 discloses a coke oven having a heating wall with only low burners on one side of a coking chamber and a heating wall having only high burners on the other side of the coking chamber. A more acceptable vertical temperature differential in the oven coking chamber is achieved by this arrangement, but the temperature of the top of the coke cake is still not well equalized with the temperature of the lower portions of the coke cake.
Accordingly, there is a need for a coke oven heating system that affords a pushed coke cake with a low vertical temperature differential through the entire height of the coke ovens.
There is also a need for a coke oven heating system that produces in an acceptable, relatively short coking time a coke cake having a low vertical temperature differential.